The use of multiple transmit/receive antennas in wireless networks promises mitigation of interference and improved spectral efficiencies through focusing signals along a designated direction (directional beamforming or focusing), or on an intended receiver (location-or spot-focusing). Compared to single-antenna-to-single-antenna transmissions, transmit beamforming may yield increased range (e.g., an N-fold increase for free-space propagation), increased rate (e.g., an N2-fold increase in a power-limited regime), increased power efficiency (e.g., an N-fold decrease in the net transmitted power for a fixed received power), and/or may allow splitting a high data-rate stream into multiple lower data-rate streams. (Here, N is the number of cooperative nodes or antenna elements at the transmit side.)
Distributed coherent RF transmit beamforming is a form of cooperative communication in which two or more nodes (that is, nodes of a node array) simultaneously transmit a common message, controlling the phase and timing of their transmissions so that the transmitted signals constructively combine at an intended destination.
In the case of directional beamforming, the individual array nodes are configured as a phased array to produce a beam that is approximately collimated in a given direction, but the beam is not specifically focused to maximize power at a given location of the target receiver. Phased arrays where the locations of the individual array elements and the target receiver are known, where the array elements are interconnected with cables or other calibrated interconnections (e.g., hardwired), and where a common centralized clock/time reference can be distributed among the array elements, can be configured to operate in such directional beamforming modes.
Decentralized arrays may be much more difficult to use as coherent beamforming phased arrays, either in transmit mode or receive mode. In a decentralized array, the individual nodes are untethered devices with independent clocks, i.e., without a distributed/hardwired clock or frequency reference. Additionally, in a decentralized array the precise positional coordinates of each node may be unknown and/or varying in time. Decentralized cooperative arrays and their operation for radio frequency (RF) communications are described in several commonly-owned and related patent documents, including the following:
1. International Patent Publication WO/2012/151316 (PCT/US2012/36180), entitled DISTRIBUTED CO-OPERATING NODES USING TIME REVERSAL, filed 2 May 2012;
2. U.S. patent application Ser. No. 14/114,901, Publication Number 2014-0126567, entitled DISTRIBUTED CO-OPERATING NODES USING TIME REVERSAL, filed on 8 May 2014;
3. U.S. Provisional Patent Application Ser. No. 61/481,720, entitled DISTRIBUTED CO-OPERATING NODES USING TIME REVERSAL FOR COMMUNICATIONS, SENSING & IMAGING, filed on 2 May 2011;
4. U.S. Provisional Patent Application Ser. No. 61/540,307, entitled DISTRIBUTED CO-OPERATING NODES USING TIME REVERSAL FOR COMMUNICATIONS, SENSING & IMAGING, filed on 28 Sep. 2011;
5. U.S. Provisional Patent Application Ser. No. 61/809,370, entitled APPARATUS, METHODS, AND ARTICLES OF MANUFACTURE FOR COLLABORATIVE BEAMFOCUSING OF RADIO FREQUENCY EMISSIONS OF RADIO FREQUENCY EMISSIONS, filed on 7 Apr. 2013;
6. U.S. Provisional Patent Application Ser. No. 61/829,208, entitled APPARATUS, METHODS, AND ARTICLES OF MANUFACTURE FOR COLLABORATIVE BEAMFOCUSING OF RADIO FREQUENCY EMISSIONS, filed on 30 May 2013;
7. International Patent Application PCT/US2014/33234, entitled DISTRIBUTED CO-OPERATING NODES USING TIME REVERSAL, filed 7 Apr. 2014; and
8. U.S. patent application Ser. No. 14/247,229, entitled DISTRIBUTED CO-OPERATING NODES USING TIME REVERSAL, filed on 7 Apr. 2014.
Each of the patent documents described above is hereby incorporated by reference, including Specification, Claims (if present), Figures, Tables (if present), and all other matter. We may refer to these documents collectively as “incorporated applications” or “related patent documents.”
Several tasks may be necessary or desirable for a decentralized cooperative array of nodes to operate as a directional beamforming or spot-focusing array. First, a decentralized array may need to acquire channel information between the individual array nodes and the intended target/source, and provide a mechanism for the nodes to transmit/receive a correctly-weighted signal at each of the array nodes (or “elements,” or “members,” which terms are used interchangeably), so that beamforming or focusing is achieved to within some predetermined or variable accuracy required by the system's specification or applications.
Second, the information to be transmitted by the decentralized array to a target may need to be distributed across the array (i.e., to the individual nodes). Alternatively, when the array is used for receiving transmissions, the data may need to be collected from the different nodes of the decentralized array.
Third, some control operations may need to be performed across the array.
Fourth, the individual nodes of the decentralized array should be phase-aligned, frequency-aligned, and time-aligned, to enable the array to operate in a coherent manner. Achieving and maintaining such alignment/synchronization and coordination of the array nodes is important to the correct operation of the array.
Some inter-nodal communications are needed in such systems. The requirements applicable to the procedures used in the inter-nodal communications may be rather strict, especially those that are imposed by the need to achieve and maintain alignment/synchronization of the different nodes. In an array of nodes, exceeding the clock coherence limit may manifest as a random scrambling of the phases of the carrier waves utilized in the beamforming or focusing, and a failure to achieve optimal or even minimally-acceptable performance. Even with atomic clocks and with fixed locations of the nodes, the coherence limit is eventually reached, requiring re-alignment of the clocks. In sum, a method used for alignment/synchronization should be fast enough to maintain the alignment required for acceptable communication operation of the array, given the coherence specifications of the clocks of the individual nodes. Moreover, there are other factors that may shorten the time between successive re-alignments, such as the movement of the nodes and the dynamic changes in the channel responses.
Improved techniques for communications between and among nodes are desirable, in particular improved techniques for time- phase-, and/or frequency-aligning/synchronizing the nodes and maintaining their alignment/synchronization in dynamic environments. Thus, needs exist in the art for improved node-to-node communication techniques for distributed coherent communications between an array of nodes and communication apparatus external to the array; for apparatus, methods, and articles of manufacture enabling such improved communications; and for phase/frequency alignment/synchronization techniques that can be used in ad hoc nodes of a distributed array for coherent communications.